A fuel assembly in a boiling-water nuclear reactor comprises a long tubular container. The container is often made with a rectangular or square cross section and is open at both ends to make possible a continuous flow of coolant through the fuel assembly. The fuel assembly comprises a large number of equally long tubular fuel rods, arranged in parallel in a defined typically symmetrical, pattern. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To allow optimum coolant optimum flow along the fuel rods, it is important to keep these at a distance from each other and prevent them from bending or vibrating when the reactor is in operation. If the flow of coolant around a fuel rod is prevented, so-called dryout may occur on the surface of the fuel rod, which may result in damage of the fuel rod. To secure the flow of coolant along the fuel rods, a plurality of spacers are distributed longitudinally along the fuel assembly; however, each new spacer contributes to the pressure drop across the fuel assembly. To minimize the risk of dryout to the fuel assembly the flow of coolant is maintained at a fixed margin, the dryout margin, designed to exceed the coolant flow at which dryout occurs under the relevant conditions.
A fuel assembly for a pressurized-water nuclear reactor has, in principle, the same construction as a fuel assembly for a boiling-water nuclear reactor except that the fuel rods are not enclosed by any tubular container and that their number is greater.
Normally, a fuel assembly comprises components for retaining the elongated elements, such as spacers, top tie plate and bottom tie plate. These components are normally made of metallic materials, preferably of zirconium alloys or so-called superalloys based on nickel. The resilient clamping force which the retaining components apply to the fuel rods decreases, relaxes, during operation of the reactor as a consequence of the metallic material mechanically degrading, under the conditions prevailing in the fuel assembly during operation. In addition, corrosion, erosion and abrasion damage arise under conditions which prevail in the fuel assembly during operation of the nuclear reactor. The degradation of the retaining components, as well as the occurrence of abrasion, erosion and corrosion damage, may be predicted. When dimensioning these components, the degradation is taken into consideration by oversizing the thickness of the material of the components. This oversizing of the thickness of the material of the components results in an increase of the pressure drop across the fuel assembly.
The object of the present invention is therefore to provide a device for retaining elongated elements such as fuel rods in a fuel assembly for a light-water nuclear reactor. The device makes possible an increased operating margin, that is, a margin with respect to dryout as a consequence of too low a coolant flow such that the components included in the device exhibit both improved mechanical properties, including a reduced relaxation of the clamping force which is applied to the elongated elements, and an improved resistance to damage caused by corrosion, erosion and/or abrasion.